In the period since September, 2001, X-Ray Computerized Tomography (CT) has been used extensively to search for explosive materials concealed in airline baggage. The method works by measuring the “CT number” of objects contained in a suitcase. The CT number is essentially a measure of the attenuation per unit length of x-rays (with a given energy distribution) in the material comprising each object. The CT number can then be used to identify the material. As a matter of definition, “CT number,” as used herein and in any appended claims, will refer to a measure of x-ray attenuation, conventionally quoted relative to the attenuation of water.
For organic materials, the CT number is essentially a measure of the electron density of the material, which in turn, is proportional to the mass density. X-Ray CT systems are therefore able to measure the mass density of concealed materials. Explosive materials tend to have mass densities which lie in the range of about 1.2-1.7 grams per cubic centimeter (g/cc). Since x-Ray CT systems reconstruct the contents of a container in three dimensions, the volume of each concealed object is also determined. Combining this information with the density yields the mass of each object. By selecting objects with a minimum size and mass which have a density between 1.2 and 1.7 g/cc, explosive threats can automatically be detected in the container, and an alarm sounded.
Disadvantages of x-Ray CT systems include their size and cost. Both the size and cost arise largely because of the rapidly rotating gantries on which the x-ray source and detector arrays are mounted.
U.S. Pat. No. 5,930,326, entitled “Side Scatter Tomography System,” described a method for detecting radiation scattered at essentially 90 degrees out of a raster-scanning pencil beam of x-rays, as detected by one or more arrays of segmented and collimated detector arrays. The intensity distribution of the side-scattered radiation is then used to reconstruct (in three dimensions) the organic objects concealed within a container. That patent is incorporated herein by reference.
The concept of Scatter Attenuation Tomography (SAT) has been previously described in U.S. Pat. Nos. 7,551,718 and 7,924,979, both of which are incorporated herein by reference. The '718 and '979 patents make use of one or more collimated scatter detectors to measure the attenuation of scatter within a concealed object or material. For example, a recent application of SAT has been to identify liquids for aviation security to ensure that no flammable, explosive, or explosive precursor liquids can be brought onto aircraft. The limitation of the SAT concept described in the '718 and '979 patents is that it is essentially a point interrogation technique which characterizes the density and effective atomic number of a single volume element or “voxel” of the object being inspected. This is because the measurement is typically made in the sub-volume defined by the intersection of the x-ray beams and the field of view of a pair of collimated scatter detectors. The location of the sub-volume of interest must therefore be known beforehand, and the system must be either moved or oriented so that this volume is being interrogated. This can be problematic if the precise location of a suspect sub-volume is not known. Another limitation of the SAT technique previously described is that in order to characterize the material in more than one sub-volume, an array of collimated scatter detectors must be used, adding considerable expense and complexity.